Abstract: A process of slowing down diffusion of an element or a compound through a fluoroplastic comprising the addition of a reactive additive having reactive groups that react with the element or compound. A process of preventing degradation of the fluoroplastics PVDF and ECTFE used in, or in connection with a reactor where chlorine dioxide is produced comprising mixing the fluoroplastic with a reactive additive having reactive groups. A diffusion resistant fluoroplastic which comprises a reactive additive having reactive groups that react with an element and/or compound to which the fluoroplastic is diffusion resistant. The use of a reactive additive having reactive groups as an additive in a fluoroplastic to prevent or slow down the diffusion of an element or a compound through the fluoroplastic.